Blender mirror error on objects
As moonboots already mentioned, it sounds like your problem's issue is that your mesh's polygons are flipped the wrong way. You can easily recalculate polygon normal directions by using the [CTRL+N] shortcut. If you wish to recalculate only selected polygons, select the desired polygons in edit-mode, than use the [RMB] shortcut to bring up the edit shortcut menu, then choose the Mesh Tools -&gt; Flip Normals selection.
Some documentation.
https://all3dp.com/2/blender-recalculate-normals-simply-explained/